The Quality of Compressed Air

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It is vitally important to the user that the compressed air be of the right quality. If air that contains contamination comes into contact with the final product, rejection costs can quickly become unacceptably high and the cheapest solution can quickly become the most expensive. It is important that you select the compressed air quality in line with the company's quality policy and even attempt to judge future requirements.

Why is it important to consider the air quality?

Compressed air can contain unwanted substances, for example, water in drop or vapor form, oil in drop or aerosol form, as well as dust. Depending on the compressed air's application area, these substances can impair production results and even increase costs. The purpose of air treatment is to produce the compressed air quality specified by the consumer. When the role of compressed air in a process is clearly defined, finding the system that will be the most profitable and efficient in that specific situation is simple. It is a question, among others, of establishing whether the compressed air will come into direct contact with the product or whether, for example, oil mist can be accepted in the working environment. A systematic method is required to select the right equipment.

How do filters help?

A flter basically separates the air particles from contaminant particles. The particle-separating capacity of a filter is a result of the combined sub-capacities (for the different particle sizes) as set forth above. In reality, each filter is a compromise, as no filter is efficient across the entire particle size range. Even the effect of the stream velocity on the separating capacity for different particle sizes is not a decisive factor.

Generally, particles between 0.1μm and 0.2μm are the most difficult to separate (Most Penetrating Particle Size).2.35.png As stated above, the total capturing efficiency of a coalescence filter can be attributed to a combination of all occurring mechanisms. Obviously, the importance of each mechanism, the particle sizes for which they occur and the value of the total efficiency heavily depend on the particle size distribution of the aerosol, the air speed and the fiber diameter distribution of the filter media.

Oil and water in aerosol form behave similar to other particles and can also be separated using a coalescing filter. In the filter these liquid aerosols coalesce to larger droplets that sink to the bottom of the filter due to gravitational forces. The filter can separate oil in aerosol as well as in liquid form. However, oil in liquid form will, due to the inherent high concentration, result in high pressure drop and oil carry-over. If oil in vapor form is to be separated, the filter must contain a suitable adsorption material, usually activated carbon.

All filtering inevitably results in a pressure drop, which is an energy loss in the compressed air system. Finer filters with a tighter structure cause a higher pressure drop and may get clogged more quickly, which demands more frequent filter replacement and consequently higher maintenance costs.

The quality of the air in regards of the amount of particles and the presence of water and oil is defined in ISO 8573-1, the industry standard for air purity. To eliminate the risk of air contamination in a critical process, it is recommended that only compressed air classified as Class 0 be used. Additionally, filters must be dimensioned so that they not only handle the nominal flow properly, but also have a larger capacity threshold in order to manage some pressure drop due to a certain amount of blockage.